A Treasury analysis warns the state pension would be less affordable in a
separate Scotland because there would be fewer workers to fund benefits for
each OAP.

The state pension would be less affordable in an independent Scotland unless it could attract almost half a million more immigrants, according to a “comprehensive” Treasury analysis of its prospective finances.

Civil servants have calculated that Scotland would need the population increase over the next 20 years – the equivalent of a city the size of Edinburgh – because there are fewer workers north of the Border paying taxes to fund each OAP’s pension.

A separate Scotland would have to more than treble its projected annual net migration from 7,000 to 24,000 if the funding gap relative to the UK was to be closed, it said.

Without a steep increase in immigration, actuaries have warned that each worker would have to pay hundreds of pounds more in tax to make good the shortfall.

An independent Scotland’s more rapidly ageing population is one of five issues the Treasury has examined as part of an analysis, being published next week, which will project its finances between 2016 and 2036.

The document will also consider the impact of declining North Sea oil revenues, the price tag of creating a new state and estimate that only three of the SNP’s spending promises would be £1.6 billion per year.

However, its most high-profile element will be the first official calculation of the ‘Union dividend’ – how much better off the average Scot would be if there is a No vote in September’s referendum.

Treasury sources said they decided to conduct the most “comprehensive” analysis yet of a separate Scotland’s finances after the Scottish Government’s 670-page White Paper on independence included only a single page of figures covering one financial year.

Leading actuaries have warned the state pension would be less affordable because Scotland’s "dependency ratio" – the number of workers paying taxes to fund each OAP's state pension – would be worse than the UK's.

The White Paper suggested increasing immigration to help meet the pension funding shortfall but gave no indication about the scale of the influx that would be required.

Alex Johnstone, a Tory MSP, said migrant workers play a vital role in Scotland’s economy but warned: “Under the SNP's hare-brained separation plans, we would need an open-door approach just to have a fighting chance of balancing the books.

"That would require a different immigration policy to the rest of the UK, which would without question mean border controls were an absolute necessity."

Gregg McClymont MP, Labour's pension spokesman, said: "Pooling and sharing our resources across the whole of the UK means pensions are more affordable in Scotland.

"The nationalists tell us they will plug the pensions gap in a separate Scotland by increasing immigration but have failed to set out any plan."

Using Office for National Statistics data, the Treasury has calculated that by 2035 the UK will have around 370 OAPs for every 1,000 workers paying taxes to fund their state pension if there are low levels of immigration over the next 20 years.

But a separate Scotland with “low” net migration of 7,000 people per year would have almost 400 pensioners per 1,000 workers.

Although the dependency ratio will increase in the remainder of the UK over the same period, the analysis found that the more rapid rise in Scotland would mean 24,000 more immigrants per year would be required to close the funding gap.

Official figures show that Scotland saw net annual migration gains of at least 18,600 per year between 2003/04 and 2010/11, a period that coincided with many former Eastern Bloc countries such as Poland joining the EU.

Next week’s analysis will also cast further doubt on Alex Salmond’s £1.2 billion promise to increase free child care and his promise the cost would be entirely paid using extra tax revenues from the mothers of young children who return to work.

Echoing a previous analysis by the Scottish Parliament Information Centre (SPICE), the Treasury will conclude there are not enough mothers in Scotland looking for work for this to be accurate.

Danny Alexander, the Chief Secretary to the Treasury, said: “This is typical of the nationalist elite who think they can con voters by inventing mothers that don’t exist and then assume that they will all get back into work.”

But John Swinney, the Scottish Finance Minister, said the analysis was “deeply flawed”. He added: “Net migration to Scotland is already more than 10,000 a year, and independence will allow us to have a flexible policy to suit our economic needs.”